minimum number of subsequences of 'source' required to form 'target'

"""
Finds the minimum number of subsequences of 'source' required to form 'target'.
Returns -1 if 'target' cannot be formed from 'source'.
"""
def solve(source: str, target: str) -> int:
    """
    Time Complexity: O(len(source) * len(target))
    Space Complexity: O(1)
    """
    answer = 0

    # Greedy, match as many as you can
    def match_subsequence(source_idx: int, target_idx: int) -> int:
        while source_idx < len(source) and target_idx < len(target):
            if source[source_idx] == target[target_idx]:
                target_idx += 1
            source_idx += 1 # subsequence -> able to skip it
        return target_idx


    target_idx = 0
    while target_idx < len(target):
        new_target_idx = match_subsequence(0, target_idx)

        if new_target_idx == target_idx:
            answer = -1
            break
        else:
            target_idx = new_target_idx
            answer += 1

    return answer